Such devices are used, for example, as biosensors in a large variety of applications. One particular application is the detection or monitoring of binding affinities or processes. For example, with the aid of such biosensors various assays detecting the binding of target samples to binding sites can be performed. Typically, large numbers of such assays are performed on a biosensor at spots which are arranged in a two-dimensional microarray on the surface of the biosensor. The use of microarrays provides a tool for the simultaneous detection of the binding affinities or processes of different target samples in high-throughput screenings, wherein large amounts of target samples like molecules, proteins or DNA can be analysed quickly. For detecting the affinities of target samples to bind to specific binding sites (e.g. the affinities of target molecules to bind to different capture molecules), a large number of binding sites is immobilised on the surface of the biosensor at spots which can be applied, for instance, by ink-jet spotting. Each spot forms an individual measurement zone for a predetermined type of capture molecules. The affinity of a target sample to a specific type of capture molecules is detected and is used to provide information on the binding affinity of the target sample.
A known technique for detecting binding affinities of target samples uses labels which are capable of emitting fluorescent light upon excitation. For example, fluorescent tags can be used as labels for labelling the target samples. Upon excitation, the fluorescent tags are caused to emit fluorescent light having a characteristic emission spectrum. The detection of this characteristic emission spectrum at a particular spot indicates that the labelled target molecule has bound to the particular type of binding sites present at the respective spot.
A sensor for detecting labelled target samples is described in the article “Zeptosens' protein microarrays: A novel high performance microarray platform for low abundance protein analysis”, Proteomics 2002, 2, S. 383-393, Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany. The sensor described there comprises a planar waveguide arranged on a substrate, and a grating for coupling coherent light of a predetermined wavelength into the planar waveguide. A further grating is arranged at that end of the planar waveguide remote from the grating for coupling the light into the waveguide. Coherent light that has propagated through the planar waveguide is coupled out of the waveguide by the further grating. The outcoupled light is used for adjustment of the coupling of coherent light of predetermined wavelength into the planar waveguide. The coherent light propagates through the planar waveguide under total reflection with an evanescent field of the coherent light propagating along the outer surface of the planar waveguide. The depth of penetration of the evanescent field into the medium of lower refractive index at the outer surface of the planar waveguide is in the order of magnitude of a fraction of the wavelength of the coherent light propagating through the planar waveguide. The evanescent field excites the fluorescent tags of the labelled target samples bound to the binding sites arranged on the surface of the planar waveguide. Due to the very small penetration of the evanescent field into the optically thinner medium at the outer surface of the planar waveguide, only the labelled samples bound to the binding sites immobilized on the outer surface of the planar waveguide are excited. The fluorescent light emitted by these tags is then detected with the aid of a CCD camera.